Locks are great, because you can lock and unlock them over and over. But that reusability is also their biggest weakness.

Any lock can be opened by anyone with the key or combination. Locks can also be picked, or destroyed with a bolt cutter and replaced with an identical lock. In any of those cases, you would never know if the package or door had been accessed—or if any of its contents had gone missing.

A seal, on the other hand, is meant for one use only. A numbered or barcoded cable seal, for instance, can’t be re-sealed or replaced. If it’s broken, you’ll know your security has been breached.

That’s an example of an indicative seal—a seal that demonstrates entry, but doesn’t prevent it. If you need to prevent entry as well, you want a barrier seal, like the one in the next example.